The term spirometry is defined as the use of a spirometer to obtain measurements of breathing capacity, however the term is used more generically to mean Lung or Pulmonary Function Testing (PFT) to acquire information about the lung volumes, flow rates and the physiological function.
The reason for assessing someone's Pulmonary Function will determine exactly what sort of PFT will be required. Sometimes just the most basic test, a Peak Flow, will suffice and provide enough information of a patient's condition for the clinician. Mostly a whole bselection of investigations will be required to provide more information to enable an accurate diagnosis or prognosis. Questionnaires can also provide useful information by asking about any symptoms, frequency of episodes and if or when Bronchodilators were last used (The use of Bronchodilators, such as Salbutamol, can often affect PFT results).
When performing spirometry the clinician must try and ensure that maximal effort is obtained at each attempt as sub-par effort will limit the clinical value of the tests. Whatever procedures are indicated they are always compared to a set of Normal Values. These values take into consideration common factors such as gender, age, height and race, on occasion other parameters are used such as Body Surface Area (BSA). There are three main sets of Normal Values:
1. European Respiratory Society (ERS) 1993 regression values, are recommended by American Thoracic Society/European Respiratory Society (ATS/ERS) Task Force as the 'Standardisation of Lung Function testing for Europeans'. These tables are the most widely used in Europe and elsewhere and were published in 1993.
2. National Health and Nutrition Examination Survey (NHANES) in it's current form of NHANES III, is recommended for ethnically appropriate individuals by ATS/ERS and is used in the Americas and elsewhere. These tables were published in 1999.
3. Global Lungs Quanjer (GLI-2012) regressions are based on the NHANES III but use an added method (Lambda-mu-sigma (LMS)) to process thenormal values. These values were published in 2008, accepted by ERS in 2010 but are not widely used.
The most commonest PFT testing that can be done is the Peak Flow. A Peak Flow Meter is a small, portable handheld device that is quite inexpensive. It's purpose is to record a person's Peak Expiratory Flow Rate (PEF or PEFR) or the fastest that a person can breathe out. This measurement is recorded in litres per minute.
Procedure: The person is instructed to inhale fully, place the tube in the mouth and exhale as hard as they can, so 'take a deep breath and blow out as hard as you can' should elicit maximum results. This procedure can repeated a number of times (best of three) until a consistent result is obtained. Many repeated attempts must be avoided due to fatigue with a resultant drop in the score.
This item is an ideal way to monitor some respiratory disorders quickly and easily in a doctors Practice or even at home, over a period of time keeping a log of the results to see if any trend is apparent, such as low scores at dawn/dusk possibly relating to pollen counts.
Spirometry is probably the most commonest clinical form of PFT. The type of machine varies and can be either a 'Bellows' type or a Strain gauge variant, although they look different they perform the same test. Spirometry records expelled volume over time and from this certain measurements may be taken.
Procedure: The person is instructed to take a deep breath and then to blow out as hard as they can and to continue to empty their lungs until told to stop (usually 6 seconds). In some cases a longer period is required for subjects to empty their lungs completely and care must be taken when this is required, especially the elderly and children.
A slight amendment to the procedure is required if the Vital Capacity is to be recorded as apposed to the Forced Vital Capacity, in this case the subject is instructed to take a deep breath and then in their own time, without forcing the air, the empty their lungs completely and then stop. With this technique people can usually empty their lungs for up to 12 seconds.
Peak Expiratory Flow (PEF) can be recorded as the calculated fastest velocity of exhaled breath.
Forced Expiratory Volume in 1 second (FEV1) is calculated as the volume that was expired at 1 second once breathing out was commenced. Further FEV measurements can be recorded but these are not done routinely such as FEV0.5, FEV3 and FEV6 (½, 3 and 6 seconds respectively).
Forced Vital Capacity (FVC) and Vital Capacity (VC), this records total volume of air expelled with slightly different techniques. If there is a difference between VC and FVC it usually indicates a collapse of the small airways.
FEV1/FVC Ratio (Tiffenau-Pinelli Index) is a calculation comparing the FEV1 to the FVC and expressed as a percentage.
Forced Expiratory Flow at 25%-75% (FEF25-75) also called the Maximum Mid-Expiratory Flow calculates the ratio of expired flow between 25% volume and 75% volume. This measurement was used to aid detection of obstruction in the small airways but has since been shown to correlate with the conventional measurements of FEV1, FVC, PEF and Tiffenau-Pinelli Index (http://dx.doi.org/10.1183/09031936.00128113)
The results of this should be able to differentiate between normal, obstructive or restrictive spirometry.
Flow Volume Loop (Shortened to Floop or Flow Loop)
As previously explained regular spirometry plots volume of air exhaled (at maximum effort) against time, a flow loop plots the rate of expiration (usually as litres per second) against volume (litres) and then also records the same parameters on inhalation. This data is represented graphically and forms a 'loop' (see diagram). There are multiple measurements that can be recorded from this technique and these are listed below.
Procedure: The initial procedure is the same as for regular spirometry however when the air is fully expelled from the lungs (the flow rate reduces to zero) the subject is then instructed to fully inspire (breath in) maximally at maximum effort. The test is complete once the inspiration is finished, noted again by the flow rate reducing to zero.
To the trained eye the shape of the flow loop can tell immediately if the test has been performed correctly and can illustrate various pathologies. The peak expiration is recorded in less than half a second and then the flow rate reduces until it reaches zero, this is the point where the subject has fully expelled all the inspired air. The subject then forcefully breathes in and the second half of the flow loop is recorded until the flow rate reduces to zero and then the test is completed. A few more attempts are then made to ensure consistency in the test results.
Peak Expiratory Flow Rate (PEFR) is the maximum rate at which the air is expelled, this is thought to indicative of the air expelled from the large airways (trachea & main bronchi).
Forced Expiratory Flow at 25% (FEF25) expresses the flow rate at 25% of the total expired volume. Assuming maximal effort is given this measurement can give some information on the medium to large sized bronchi.
Forced Expiratory Flow at 50% (FEF50) is the flow rate when half the total capacity is expired and should be less than FEF25. This can indicate the condition of the small to medium airways.
Forced Expiratory Flow at 75% (FEF75) is the flow rate when 75% of the maximum capacity is reached, this should be less than FEF25 and FEF50. This measurement can provide information on the small airways. Most chronic pulmonary airway diseases show up in the small airways initially so the latter part of expiration is affected.
Forced Expiratory Flow 25-75% (FEF25-75) is the mean flow between these two points and can be quite important as this measurement will also decline in many respiratory diseases.
Forced Inspiratory Flow at 25%, 50% and 75% (FIF25, FIF50 & FIF75) are the flow rates of inspiration at a certain percentage of the total inspired volume. These measurements are less important in assessing small airway diseases and can provide some information on upper airway obstructions (pharynx, larynx, mouth and vocal chords).
Peak Inspiratory Flow Rate (PIFR) is the fastest flow rate on inspiration.
Forced Expiratory Time (FET) is the time taken to exhale a full breath.
Spirometry and Reversibility Testing
Obstructive airways diseases have many causes and performing reversibility testing with spirometry may provide support for a diagnosis of reversible asthma. Normal spirometry is performed and if there is a reduction in the FEV1 or the Tiffenau-Pinelli Index (FEV1/FVC ratio) then a short acting bronchodilator is administered and spirometry is repeated again. Any changes to the results are noted.
If there is an increase of >12% (at least 200ml) in the FEV1 after the bronchodilation then this indicates a reversible airflow obstruction and supports the diagnosis of asthma. It also gives an predictive indication of a good response to an inhaled corticosteroid.
The absence of a reversible airways obstruction does not exclude the diagnosis of asthma and further different testing can be done.
In some subjectively symptomatic patients there is no obvious abnormality seen in spirometry regularly and this may result in a normal result at the time of testing. Some form of challenge or provocation may be required to elicit a change and confirm the presence of asthma. This can either be done;
a. Directly (using agents that directly constrict smooth airway muscle) using histamine or metacholine
b. Indirectly (activating mast cells to release mediators that affect the smooth muscle) such as exercise or inhaling hypertonic saline.
Whichever method is used the spirometry is performed before any provocation and then the challenge is given using an approved protocol either for increasing concentrations of histamine/metacholine/saline or a set exercise protocol and then the spirometry is repeated on numerous occasions either during the increasing dosage or at intervals after the exercise. Changes in the FEV1 will be noted and a calculation of the maximal drop in FEV1 post provocation is recorded and if there is a reduction a bronchodilator is given and the post bronchodilator spirometry results are recorded.
Test of Eosinophilic Inflammation/Exhaled Nitric Oxide Levels
The exhaled nitric oxide levels may be monitored over a period of time and in conjunction with sputum eosinophillia counts can have a high specificity and sensitivity to asthma but is more useful in assessing the severity of asthma and/or the response to treatment. These investigations are somewhat more lengthy and are not routinely used in the diagnosis/treatment of asthma in USA/UK.
© 2016 by Lee Boswell